Transconductance varying circuit of transconductor circuit, varying bandwidth filter circuit using the same and digital tuning circuit of transconductor-capacitor filter

ABSTRACT

The present invention relates to a circuit for varying bandwidth of transconductance-capacitor filter by controlling transconductance of a transconductance circuit, and a digital tuning circuit of transconductor-capacitor filter. A transconductor of an embodiment of the present invention comprises a first and second amplifying devices; a resistor; a first and second bias current sources; and transconductance varying circuit. A tuning circuit of another embodiment of the invention relates to a digital tuning circuit comprising a transconductor that outputs current proportional to input voltage and a varying capacitance that is connected with output node of transconductor and between grounds and varies its capacitance depending upon the level of control signal. The digital tuning circuit further comprises a transconductor, a comparator, a counter, a varying capacitor, means for maintaining the output voltage of the transconductor as substantial zero for first period, and means for inputting input voltage to the transconductor for second period and means for inputting the output voltage of the transconductor to input node of the comparator.

TECHNICAL FIELD

The present invention relates to a communication system. In particular,it is directed to a varying bandwidth gm-C filter using varying circuitof transconductance of transconductor and gm-C filter using digitaltuning circuit.

BACKGROUND OF THE INVENTION

The gm-C filter which is a filter comprising a transconductor and acapacitor is a widely used device for reconstructing received signal ina communication apparatus and for anti-aliasing of transmitted signal.The transconductor means a circuit for outputting current proportionalto applied input voltage thereto. The output current is calculated withmultiplying the applied voltage by transconductance gm.

In a transconductor-capacitor filter, transconductance gm is animportance parameter for determining output current of transconductorand cut-off frequency of a filter. The transconductance gm is determinedin accordance with the transconductor.

FIG. 1 shows a circuit diagram of a conventional transconductor.

As shown in FIG. 1, the transconductor comprises first and second NMOStransistors MN 11 and MN 12, first and second bias current sources IB11and IB12 and a degeneration resistor R11.

The relation between the components will be described with reference toFIG. 1.

The gates of the first and second NMOS transistors MN11 and MN12 formfirst and second input nodes Vin+ and Vin−, respectively. The drains ofthe transistors form first and second output nodes Iout1 and Iout2,respectively. Bias current is supplied to the sources of the first andsecond NMOS transistors MN11 and MN12 from the first and second biascurrent sources IB11 and IB12. The degeneration resistor R11 is providedbetween the sources of the first and second NMOS transistors MN11 andMN12.

The first and second input voltage Vin+ and Vin− applied to the gates ofthe first and second NMOS transistors MN11 and MN12 causes current lo inthe first and second output nodes Iout1 and Iout2, which has valuecalculated with multiplying input voltage Vin by transconductance gm.The transconductance gm is determined by the degeneration resistor R11and is in inverse proportional to resistance value of R11.

The cut-off frequency of a transconductor-capacitor filter is linearlyproportional to gm/C. Thus, transconductance gm of the transconductormaintains an initial value if a passive device R11 having fixedresistance is used as shown in FIG. 1. Further, the bandwidth of thefilter has constant value.

Much recent communication system receives more than two signals havingdifferent bandwidth one another. If the above-described filter whereinbandwidth is fixed is used for the system, a plurality of filters havingdifferent cut-off frequency one another should be used. Thus, a varyingbandwidth filter has been studied in order to resolve the problem.

Much recent communication system receives more than two signals havingdifferent bandwidth one another. If the above-described filter whereinbandwidth is fixed is used for the system, a plurality of filters havingdifferent cut-off frequency one another should be used. Thus, a varyingbandwidth filter has been studied in order to resolve the problem.

There is a conventional apparatus which uses MOSFET device for varyingcut-off frequency of the filter depending upon received signal, in lieuof degeneration resistor R11. In this apparatus, resistance of theMOSFET device varies by applying different control voltage to the gatesof the MOSFET device in accordance with received signal. The resistanceof the degeneration resistor can vary in accordance with the receivedsignal, thereby varying transconductance gm of the transconductor.

However, in the conventional art, the range of reluctance that can beobtained from control of voltage applied to the gate of MOSFET device isvery limited, thereby limiting the variation of cut-off frequency of thefilter. Further, the non-linearity of MOSFET device causes performancedeterioration of the filter.

In a transconductance-capacitor filter, the transconductance gm variesup to 50% from design value in accordance with temperature, variation ofpower voltage and manufacturing process and the like. Thus, thetransconductance-capacitor filter should employ tuning circuit thatmaintains cut-off frequency as being constant.

The conventional tuning circuit is usually an analog tuning circuit thatcontrols transconductance gm of transconductor so as to maintain thecut-off frequency of the filter as being constant.

However, there is problem in that the clock used in the analog tuningcircuit causes noise and the circuit operates continuously even whentuning is unnecessary. These are reasons for wasting power anddeteriorating filter performance.

U.S. Pat. Nos. 5,245,646 and 5,914,633 disclose a digital tuning circuitfor resolving the problems of the analog tuning circuit.

The tuning circuit disclosed by the patents is a tuning circuit of RCactive filter which maintains cut-off frequency as being constant withdigital control of RC time constant. That is, capacitor of the RC activefilter is embodied as capacitor array and digital codes controls on-offof the capacitors of the array so as to compensate the variation of timeconstant caused by operating condition, temperature and the like.

However, the conventional digital tuning circuit is limited to RC activefilter. It is difficult that the circuit is used for tuning circuit oftransconductor-capacitor filter which should detect/compensate thevariation of transconductance gm.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide atransconductor circuit for varying transconductance gm linearly

It is another object of the present invention to provide atransconductor-capacitor filter for varying bandwidth with varyingtransconductance of transconductor.

It is further object of the present invention to provide a digitaltuning circuit for maintaining cut-off frequency of atransconductor-capacitor filter.

It is yet another object of the present invention to provide a tuningcircuit which does not operate when tuning is not necessary.

In order to achieve the object, the transconductance varying circuit ofa transconductor circuit in accordance with the present inventioncomprises (1) a first and second amplifying device having first, secondand third nodes, wherein amount of current flowing to said second nodefrom said first node is controlled in proportion to voltage applied tosaid third node; (2) a resistor being connected between said secondnodes of said first and second amplifying devices; (3) a first andsecond bias current source being connected with said second nodes andgrounds of said first and second amplifying devices; and (4) at leastone resistor and at least one switching means wherein said resistor andswitching means are serially connected with each other to be connectedwith said second nodes of said first and second amplifying device.

The transconductor-capacitor filter having a transconductor andcapacitor of the present invention has transconductor which comprises(1) a first and second amplifying devices having a first, second andthird nodes wherein amount of current flowing to said second node fromscidfirst node is controlled in proportion to voltage applied to saidthird node; (2) a resistor being connected between said second nodes ofsaid first and second-amplifying devices; (3) a first and second biascurrent source being connected with said second nodes and grounds ofsaid first and second amplifying-devices; and (4) at least one resistorand at least one switching means wherein said resistor and switchingmeans are serially connected with each other to be connected with saidsecond nodes of said first and second amplifying device.

The first and second amplifying devices are MOSFET and the first, secondand third nodes are drain, source, and gate, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the present invention will be described indetail with reference to attached drawings.

FIG. 1 shows a circuit diagram of a transconductor of the conventionaltransconductor-capacity filter.

FIG. 2 shows a circuit diagram of a transconductor according to anembodiment of the present invention.

FIG. 3 shows a circuit diagram for describing control method of atransconductor varying circuit according to an embodiment of the presentinvention.

FIG. 4 shows a circuit diagram of tuning circuit of atransconductor-capacitor filter according to an embodiment of thepresent invention.

FIG. 5 shows a waveform of the voltage applied to first to fifthswitching means SW51 to SW55 in the tuning circuit shown in FIG. 4 and awaveform of output voltage from the transconductor according to theapplied voltage.

FIG. 6 shows a circuit diagram of the modified first and second varyingcapacitors according to another embodiment of the present invention, inthe tuning circuit shown in FIG. 4.

DETAILED DESCRIPTION

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the attached drawings.

The First Embodiment

FIG. 2 shows a circuit diagram showing a transconductor of an embodimentof the present invention.

The transconductor of this embodiment uses MOSFET amplifying device. Theamplifying device comprises a gate, a source and a drain. MOSFETamplifying device has a characteristic wherein the amount of current andthe direction of current which flows to the source from the drain orvice versa are determined in accordance with voltage and polarity of thevoltage applied to the gate. Such an amplifying device includes abipolar junction transistor (BJT), a junction field effect transistor(JFET), a metal-oxide semiconductor field effect transistor (MOSFET), ametal semiconductor field effect semiconductor and the like.

Of the above-listed devices, MOSFET will be described mainly in thebelow. However, the spirit of the present invention is not limited toMOSFET and extends to all of the devices operating complementarily.Further, the following description relates to N-MOSFET. However, it isalso clear in the related field that the present invention can beapplied to P-MOSFET.

As shown in FIG. 2, the transconductor of an embodiment of the presentinvention comprises first and second amplifying devices MN21 and MN22,first and second bias current sources IB21 and IB22, resistor R21 andtransconductance varying circuit 2100. Transconductance varying circuit2100 comprises one or more resistor. The circuit controls degenerationresistance of the transconductor, thereby varying transconductance gm.

The connection among the components will be described with reference toFIG. 2.

The gates of first and second amplifying devices MN21 and MN22 formfirst and second input nodes Vin+ and Vin−, respectively. The drains ofthe devices form first and second output nodes Iout1 and Iout2,respectively. First and second bias current sources IB21 and IB22 supplybias current to the sources of first and second amplifying devices MN21and MN22, respectively. Resistor R21 is provided between the sources offirst and second NMOS transistors MN21 and MN22.

Transconductance varying circuit 2100 is provided between the sources offirst and second amplifying devices MN21 and MN22. One end of firstresistor Rc21 is connected with the source of first amplifying deviceMN21 through first switching means SW21 while the other end of theresistor is connected with the source of second amplifying device MN22through second switching means SW22. One end of second resistor Rc22 isconnected with the source of first amplifying device MN21 through thirdswitching means SW23 while the other end of the resistor is connectedwith the source of second amplifying device MN22 through fourthswitching means SW24.

In transconductance varying circuit 2100 of the embodiment,substantially same switching means are connected to both ends of firstand second resistors Rc21 and Rc22. However, this is for matchingcharacteristic of transistors. It should be understood that the spiritof the present invention is not limited to the number of switching meansconnected to first and second resistors Rc21 and Rc22. That is, it ispossible that only one switch is connected to one end of first andsecond resistors Rc21 and Rc22 while the other ends of first and secondresistors Rc21 and Rc22 are directly connected to the source of theamplifying device.

FIG. 2 shows first and second resistors Rc21 and Rc22 and first tofourth switching means SW21 to SW24 for convenience of description.However, more resistors and switching means may be connected between thesources of first and second amplifying devices. In an embodiment,transconductance varying circuit 2100 can be formed only using firstresistor Rc21 and first and second switching means SW21 and SW22.

The operation of the transconductor of the embodiment will be describedin detail.

First and second amplifying devices MN21 and MN22 controls current loflowing in drain with input voltage Vin+ and Vin− applied to each gate.

Transconductance varying circuit 2100 controls on off of first to fourthswitching means SW21 to SW24, thereby varying degeneration resistance ofthe transconductor. Further, first and second switching means SW21 andSW22 and third and fourth switching means SW23 and SW24, operate as apair, respectively.

As described in the above, transconductance gm of the transconductor isin inverse proportion to the degeneration resistance value. Thus, thetransconductance gm can vary by controlling the resistance oftransconductance varying circuit 2100. If all of the switches are open,minimum transconductance gm is obtained since only resistor R21 is thedegeneration resistor. On the other hand, if all of the switches areclosed, maximum transconductance gm is obtained.

Further, since cut-off frequency of a transconductor-capacitor filter isproportional to transconductance gm of transconductor, the cut-offfrequency can be controlled by varying transconductance gm.

FIG. 3 shows a circuit diagram for the purpose of describing controlmethod of transconductance varying circuit 3100 of an embodiment of thepresent invention.

Transconductance varying circuit 3100 shown in FIG. 3 comprises fourresistors Rc31 to Rc34 and eight switching means SW31 to Sw39.

Switching means are connected to both ends of each resistor and areconnected between the sources of first and second NMOS transistors MN31and MN32. The same control signal is applied to the switching meansconnected to both ends of the same resistor. That is, the switchingmeans connected to both ends of first to fourth resistors Rc31 to Rc34operates as a pair and is on and off simultaneously.

Four bit control signals are necessary in the event that transconductorvarying circuit 3100 are constituted with four resistors Rc31 to Rc34and eight switching means SW31 to SW38. On-off of each switching meansSW31 to SW39 is determined in accordance with the level of appliedcontrol signals ctr1 to ctr4. In transconductor varying circuit 3100 ofone embodiment of the present invention, each switching means SW31 toSW38 is closed when the applied signal is high level and while it isopen when the applied signal is low level. For example, when appliedcontrol signal is 1001, first and second switching means SW31 and SW32and seventh and eighth switching means are closed, thereby constitutingdegeneration resistor with parallel connection of resistor Rc, firstresistor R31 and fourth resistor R34.

Therefore, transconductance gm can be selected from sixteen values iftransconductance control circuit 3100 is embodied as shown in FIG. 3 andcut-off frequency of the transconductor-capacitor filter is determinedwith the selected gm.

The tuning circuit according to another embodiment of the presentinvention relates to a tuning circuit of filter comprising atransconductor outputting current proportional to input voltage and avarying capacitor that is connected between the output node and groundnode of the trasconductor. The capacitance varies depending upon thelevel of control signal. The tuning circuit comprises a transconductoroutputting current proportional to applied input voltage; a comparatorthat compares input voltage applied to the input node with the inputvoltage, and thereafter outputs signal to up-signal output node if thevoltage of input node is higher and outputs signal to down-signal outputnode otherwise; a counter that is connected to the up-signal output nodeand down-signal output node, and reduces/rises the level of the outputsignal by a predetermined amount responding to up-signal anddown-signal, with outputting control signal to the varying capacitor ofthe filter; a varying capacitor whose capacitance varies in accordancewith the level of output signal of the counter, with being connected tothe output node and the ground node of the transconductor; means forrendering the output voltage of the transconductor substantially zerofor first period; means for inputting the input voltage to thetransconductor of the tuning circuit for second period; and means forinputting the output voltage of the transconductor of the tuning circuitto the input node of the comparator for third period.

The transconductor of the tuning circuit may be embodied as beingsubstantially same as the transconductor of the filter.

The varying capacitor of the tuning circuit may be embodied as beingsubstantially same as the varying capacitor of the filter.

The tuning circuit further comprises a capacitor that is connected tothe input node of the comparator and the ground node.

The varying capacitor may comprise a main capacitor, an auxiliarycapacitor and switching means. One end of the main capacitor isconnected with one end of the switching means so as to be connected withthe output node of the transconductor while the other ends of the maincapacitor and the auxiliary capacitor are grounded.

The varying capacitor may be embodied with one or more main capacitorsand capacitor banks.

The second Embodiment

FIG. 4 shows a circuit diagram of a tuning circuit oftransconductor-capacitor filter of another embodiment of the presentinvention.

Main filter 5000 shown in FIG. 4 is a transconductor-capacitor filtercomprising a transconductor (not shown) and a varying capacitor (notshown). The transconductor of main filter 5000 outputs currentproportional to input voltage applied to output node. As described inthe above, transconductance gm varies depending upon process andenvironment variation. The varying capacitor of main filter 5000 isconnected with the output node of the transconductor and its capacitanceis controlled by control signal outputted from tuning circuit 4000. Thatis, capacitance is compensated by the variation amount oftransconductance gm of transconductor, thereby maintaining the cut-offfrequency as being constant.

Tuning circuit 4000 of further embodiment comprises a transconductor, acomparator, a counter, a varying capacitor and first to third switchingmeans. The transconductor outputs current proportional to the appliedinput voltage. The comparator compares the input voltage with the outputvoltage of the transconductor applied to the input node. If the voltageof the input node is higher, the comparator outputs signal to up-signaloutput node. Otherwise, it outputs signal to down-signal output node.The counter is connected with the up-signal output node and thedown-signal output node of the comparator. The counter increases ordecreases the level of the output signal by a predetermined amount,responding to the up-signal and the down-signal. The output signal ofthe counter is inputted to the varying capacitor of the filter ascontrol signal which controls capacitance of the varying capacitor. Thevarying capacitor of tuning circuit 4000 is connected with the outputnode of the transconductor. The capacitance is controlled by outputsignal of the counter, thereby controlling output voltage oftransconductor. The first switching means sets output voltage of thetransconductor to zero for first period. The second switching meansinputs input voltage to the transconductor for second period. The thirdswitching means inputs output voltage of the transconductor to inputnode of the comparator for third period.

Tuning circuit 4000 of the embodiment will be described in more detailwith reference to FIG. 4.

As shown in FIG. 4, tuning circuit 4000 comprises a transconductor 4100,a comparator 4300, a counter 4500, first and second varying capacitorsC41 and C42, third and fourth capacitors C43 and C44 and first to fifthswitching means SW41 to SW45. Main filter 5000 comprises one or moretransconductors and varying capacitors. Transconductor 4100 of tuningcircuit 4000 and first and second varying capacitors C41 and C42 areembodied substantially same as the transconductor of main filter 5000and varying capacitor.

Transconductor 4100 has first and second input nodes 401, 403 and firstand second output nodes 405, 407 and controls current flows in first andsecond output nodes 405, 407 with accordance to the voltage applied inboth of first and second input nodes 401, 403. That is, the outputcurrent of transconductor 4100 is proportional to the voltage applied tofirst and second input nodes 401, 403. The proportional coefficient istransconductance gm of transconductor 4100.

Comparator 4300 has first to fourth input nodes 409 to 415 and first andsecond output nodes 417, 419 and compares the voltage applied to firstand second input nodes 409, 411 with the voltage applied to third andfourth input nodes 413, 415 so as to output up-signal UP and down-signalDN to first and second output nodes 417, 419. If the voltage applied tofirst and second input nodes 409, 411 is higher than the voltage appliedto third and fourth input nodes 413, 415, up-signal UP of high level isoutputted to first output node 417. Otherwise, down-signal DN of lowlevel is outputted to second output node 419.

Counter 4500 outputs control signal that is generated by adding orreducing predetermined bits from the setting bits when comparator 4300applies up-signal UP and down-signal DN. If up-signal UP is applied, thebit of the control signal increases while if down-signal DN is appliedthe bit of the control signal reduces by the predetermined bits.

In tuning circuit 4000 of another embodiment of the present invention,the substantially same control signal is applied to first and secondvarying capacitors C41 and C42 and the varying capacitors have thesubstantially same capacitance. Further, third and fourth capacitors C43and C44 have the substantially same capacitance. That is, first andsecond varying capacitors C41 and C42 and third and fourth capacitorsC43 and C44 operate as a pair, respectively.

First and second input nodes 401, 403 of transconductor 4100 areconnected with + node and − node of input voltage Vin with second andthird switching means SW42 and SW43, respectively. First and secondoutput nodes 405, 407 are connected with first and second input nodes409, 411 of comparator 4300 with fourth and fifth switching means SW44an SW45, respectively.

Third and fourth input nodes 413, 415 of comparator 4300 are connectedwith + node and − node of input voltage Vin, respectively. Up-signal UPand down-signal DN are outputted from first and second output nodes 417,419.

First and second varying capacitors C41 and C42 are connected with firstand second output nodes 405, 407 of transconductor 4100 and ground node,respectively. Third and fourth capacitors C43 and C44 are connected withfirst and second input nodes 409, 411 of comparator 4300 and betweengrounds, respectively.

First switching means SW41 is connected between first and second outpautnodes 405, 407 of transconductor 4100.

FIG. 5 shows a waveform of voltage s1, s2 and s3 applied to first tofifth switching means SW41 to SW45 and the output voltage Vout oftransconductor 4100 therefrom.

In FIG. 2, s1 is a waveform of voltage applied to first switching meansSW41; s2 is a waveform of voltage applied to second and third switchingmeans SW42 and SW43; and s3 is a waveform of voltage applied to fourthand fifth switching means SW44 and SW45. Each switching means is inclose state when the applied voltage is high level.

As shown in FIG. 5, s1, s2 and s3 maintain high level for the firstperiod, the second period and the third period, respectively s2 and s3maintain low level while s1 maintains high level. Further, outputvoltage Vout of transconductor 4100 is substantially zero for theperiod. If s2 is high level, the output voltage Vout increasescontinuously until s2 becomes low level. That is, the second and thirdswitching means SW42 and SW43 are closed for the second period. Further,current which has a value determined by multiplying input voltage Vin bytransconductance gm, flows in the output node of transconductor 4100.Thus, the first and second varying capacitors C41 and C42 are chargedand the output voltage Vout increases. If s3 becomes high level and ifs1 and s2 become low level, the fourth and fifth switching means SW44and SW45 are closed and the output voltage Vout maintains as being fixedvoltage until s1 becomes high level.

The operation of tuning circuit 4000 of still further embodiment of thepresent invention will be described with reference to FIGS. 4 and 5.

In this embodiment, transconductor 4100 is embodied substantially sameas the transconductor involved in main filter 5000, and reflects thevariation of transconductance gm of the transconductor. That is, thevariation of transconductance gm of the transconductor involved in mainfilter 5000 can be known by monitoring the variation of transconductancegm of transconductor 4100 involved in tuning circuit 4000.

Further, the cut-off frequency of main filter 5000 can be maintained asbeing constant, by applying the substantially same control signal as thecontrol signal for compensating the variation of transconductance gm oftransconductor 4100 involved in tuning circuit 4000.

The operation of tuning circuit 4000 of yet another embodiment of thepresent invention will be described in more detail.

When the fist switching means SW1 is closed, both of the first andsecond output nodes 405, 407 of transconductor 4100 are closed, therebylowering the level of the output voltage Vout of transconductor 4100 tozero substantially. Further, the first and second varying capacitors C41and C42 are discharged.

When the second and third switching means SW42 and SW43 are closed,input voltage Vin is applied to the first and second input nodes 401,403 of transconductor 4100. Further, output current flows in first andsecond output nodes 405, 407. Thus, first and second varying capacitorsC41 and C42 are charged during the time t when second and thirdswitching means SW42 an SW43 are closed. Voltage Vout between first andsecond output nodes 405, 407 of transconductor 4100 is as follows.$\begin{matrix}{{Vout} = {{\frac{1}{C}Z_{0}^{t}{idt}} = {{\frac{1}{C}Z_{0}^{t}{{Vin} \cdot {gmdt}}} = {\frac{t}{C} \cdot {gm} \cdot {Vin}}}}} & {{Eqn}.\quad 1}\end{matrix}$

C means capacitance of first and second varying capacitors C141 and C42;i means current flowing in the output node of transconductor 4100; andgm means transconductance of transconductor 4100.

If voltage gain of transconductor 4100 is 1, transconductance gm oftransconductor 4100 is C/t. The transconductance gm is not affected byprocess variation. That is, if input voltage Vin of transconductor 4100is same as output voltage Vout, transconductance gm of transconductor4100 is determined by capacitance C of first and second varyingcapacitors C41 and C42 and time t when second and third switching meansSW42 and SW43 are closed, without being affected by process variationand operation environment.

Thus, if the period t of voltage that is applied to second and thirdswitching means SW42 and SW43 in the above formula is set as C/gm,thereby making voltage gain of transconductor 4100 1, transconductancegm of transconductor 4100 cannot be affected by process variation andthe like.

However, even if input voltage Vin is initially set so as to be same asoutput voltage Vout, output voltage Vout may have different from inputvoltage Vin in accordance with the variation of transconductance gm.Comparator 4300 compares input voltage Vin with output voltage Vout todetect the variation of transconductance.

In the above formula, output voltage Vout of transconductance 4100 is ininverse proportion to capacitance C of first and second varyingcapacitors C41 and C42. Thus, the variation of output voltage Voutcaused by transconductance gm with controlling capacitance of first andsecond varying capacitors C41 and C42.

When fourth and fifth switching means SW44 and SW45 are closed, thirdand fourth capacitors C43 and C44 have common charge with first andsecond varying capacitors C41 and C42. Further, output voltage Vout oftransconductor 4100 is applied to input nodes 409, 411 of comparator4300. Comparator 4300 compares voltage Vout applied to first and secondinput nodes 409, 411 with input voltage Vin applied to third and fourthinput nodes 413, 415 to output up-signal UP and down-signal DN towardfirst and second output nodes 417, 419. Specifically, if output voltageVout of transconductor 4100 is higher than input voltage Vin, up-signalis outputted. Otherwise, down-signal is outputted.

Counter 4500 controls bits of control signal which is outputted inaccordance with up-signal UP and down-signal DN from comparator 4300,and applies the control signal to first and second varying capacitorsC41 and C42, thereby controlling capacitance C of first and secondvarying capacitors C41 and C42. The control signal is also applied tovarying capacitor of main filter 5000 so that transconductance gm of themain filter is controlled with the same way as tuning circuit 4000.

FIG. 5 shows a circuit diagram of first and second varying capacitor oftuning circuit 4000 shown in FIG. 4.

As described in the above, first and second varying capacitors C41 andC42 have the substantially same capacitance due to the same controlsignal and are embodied with the same form. For convenience, only firstvarying capacitor C41 will be described.

As shown in FIG. 6, first varying capacitor C41 comprises one maincapacitor C61, four auxiliary capacitors cb61 to cb64, and fourswitching means sw61 to sw64. One end of main capacitor C61 is connectedwith one end of first to fourth switching means sw61 to sw64 so as to beconnected with first output node 405 of transconductor 4100 with theother end being grounded. The other end of first to fourth switchingmeans sw61 to sw64 is connected with one end of first to fourthauxiliary capacitors cb61 to cb64. The other end of first and fourthauxiliary capacitors cb61 to cb64 is grounded. The auxiliary capacitorsand switching means may be embodied with various forms. They can beembodied with capacitor bank that is widely used currently.

First varying capacitor shown in FIG. 6 comprises one main capacitorC61, four auxiliary capacitors cb61 to cb64 and four switching meanssw61 to sw64. It is clear to a person skilled in the art that the numberof the components can vary in accordance with embodiment. Further, thenumber of bits of the control signal from counter 4500 is determined bythe number of the auxiliary capacitors serially connected one anotherand of switching means. That is, in the event that as shown in FIG. 6four capacitors cb61 to cb64 and four switching means sw61 to sw64 areused, a counter that can output control signal with four or more bits isnecessary. The control signal is outputted as binary form (0000˜1111)between 0 and 15.

The control signal from counter 4500 is applied to first to fourthswitching means sw41 to sw44. The switching means is on or off inaccordance with the level of the control signal. In tuning circuit 4000,first to fourth switching means sw41 to sw44 are closed when the controlsignal that is applied to each switch is high level i.e., 1 while theyare open otherwise. For example, if the control signal 1001, first andfourth switches sw41 to sw44 are closed while second and third switchessw42 and sw43 are open.

In tuning circuit 4000 of another embodiment of the present invention,standard control signal is applied to first and second capacitors C41and C42 in initial state. The standard control signal determines initialcapacitance Co of first and second varying capacitors C41 and C42. Forexample, if the standard control signal is set as 0110, the initialcapacitance Co is determined with summation of main capacitor C61 andsecond and third auxiliary capacitor cb62 and cb63. Up-signal UP anddown-signal DN increase and decrease bits of the standard controlsignal, respectively whenever it is applied, thereby controllingcapacitance first and second varying capacitors C41 and C42.

Therefore, tuning circuit 4000 of one embodiment of the presentinvention compares input voltage Vin of transconductor 4100 with outputvoltage Vout to detect variation of transconductance gm, which can becompensated with control of capacitance of first and second varyingcapacitors C1 and C2. Further, the same signal as the control signalapplied to tuning circuit 4000 is applied to compensate the variation oftransconductance gm of main filter 4000.

As described in the above, first and second varying capacitors C41 andC42 can be embodied with one or more main capacitors, auxiliarycapacitor and switching means. Their capacitance is controlled withcontrol signal from counter 5000.

Further, tuning circuit 4000 of the present invention is a digitaltuning circuit using counter 4500, which can be designed so as not tooperate after a predetermined time necessary for tuning lapses. Further,the effect that tuning circuit makes to a main filter can be minimizedcontrary to the conventional analog tuning circuit, thereby savingelectric power.

INDUSTRIAL APPLICABILITY

According to the transconductor circuit of the present invention,transconductance can be linearly varied with transconductance varyingcircuit using resistance switching.

Further, cut-off frequency of transconductor-capacitor filter can bevaried within wide bandwidth, by varying transconductance oftransconductor.

In a transconductor-capacitor filter, the variation of transconductancegm can be compensated with control of capacitance of capacitor andcut-off frequency of the filter can be maintained as being constant.

If tuning is unnecessary since a predetermined time passes after tuning,the tuning circuit can be made not operate, thereby reducing effect thattuning makes to main filter performance.

1-3. (canceled)
 4. A tuning circuit of filter having a transconductoroutputting current proportional to input voltage and varying capacitorbeing connected with output nodes and between grounds of saidtransconductor and having capacitance varying in accordance with levelof control signal, said tuning circuit comprising: a transconductoroutputting current proportional to applied input voltage; a comparatorcomparing voltage applied to input node to output signal to up-signaloutput node if the voltage of said input node is higher and to outputsignal to down-signal output node otherwise; a counter being connectedwith said up-signal output node and said down-signal output node of saidcomparator, increasing or decreasing the level of output signal by apredetermined amount in response to said up-signal and said down-signal,said output signal being inputted as control signal to varying capacitorof said filter; a varying capacitor being connected with output node ofsaid transconductor and between grounds of said tuning circuit,capacitance thereof varying in accordance with the level of the outputsignal of said counter; means for maintaining the output voltage of thetransconductor of said tuning circuit as substantial zero for firstperiod; means for inputting said input voltage to said transconductor ofsaid tuning circuit for second period; and means for inputting outputvoltage of said transconductor of said tuning circuit to the input nodeof said comparator for third period.
 5. The tuning circuit of claim 4wherein said transconductor of said tuning circuit is embodied with thesubstantially same form as the transconductor of said filter.
 6. Thetuning circuit of claim 4, wherein the varying capacitor of said tuningcircuit is embodied with the substantially same form as the varyingcapacitance of said filter.
 7. The tuning circuit of claim 4, furthercomprising a capacitor being connected with said input node and betweengrounds of said comparator.
 8. The tuning circuit of claim 4, whereinsaid varying capacitor comprises a main capacitor, auxiliary capacitorand a switching means, one end of said main capacitor being connectedwith one end of said switching means to connects output node of saidtransconductor, the other end of said switching means being connectedwith one end of said auxiliary capacitor, the other end of said maincapacitor and said auxiliary capacitor being grounded.
 9. The tuningcircuit of claim 4, wherein said varying capacitor is embodied with oneor more main capacitors and capacitor bank.